I can't help you clear that up, as I don't understand it either. Some "rare" skillbooks can be bought in your homestead from the elementals. It might be my imagination, but I think I seen whirlwind skillbook on the fire elemental.
Also there are some mods out there that allow the purchase of all skillbooks, not sure if modding is easier on a mac though.
With kind regards,
Rashar.
